A silicon-yttrium sol-gel surface coating of the present invention is applied to metal, especially through a waterborne reactive sol, to provide a stable oxide surface that enhances organic matrix resin adhesion with the goal of achieving adhesion equivalent to conventional wet-chemical surface treatment methods.
Conversion coatings for titanium, aluminum, or other metals are electrolytic or chemical films that promote adhesion between the metal and an organic adhesive resin, especially for adhesive bonding. Anodizing is a conventional process for making electrolytic films by immersing titanium or its alloys in chromic acid or an alkaline earth hydroxide or aluminum in chromic, sulfuric, or phosphoric acid. Anodizing produces a porous, microrough surface into which primer (a dilute solution of adhesive) can penetrate. Adhesion results primarily from mechanical interlocking between the rough surface and the primer. Chemical films include either a phosphate-fluoride conversion coating or films made with alkaline peroxide or other alkaline etchants for titanium substrates and alodine films for aluminum substrates.
Using strong acids or strong bases and toxic materials (such as chromates) in immersion tanks, these surface treatment processes present environmental concerns. They require significant amounts of water to rinse excess process solutions from the treated parts. The rinse water and spent process solutions must be treated to remove dissolved metals prior to their discharge or reuse. Removing the metals generates additional hazardous wastes that are challenging to cleanup and to dispose. They greatly increase the cost of using the conventional wet-chemical processes. A process that will produce adhesive bonds with equivalent strength and environmental durability to these standard processes without generating significant hazardous wastes while eliminating the use of hazardous or toxic materials would greatly enhance the state-of-the-art. The present invention is one such process. In addition, the sol of the present invention can be applied by spraying rather than by immersion. It, therefore, is more readily adapted to use for field repair and maintenance.
Surface anodizing chemically modifies the surface of a metal to provide a controlled oxide surface morphology favorable to receive additional protective coatings, such as primers and finish paints. The surface oxides function as adhesion coupling agents for holding the paint lacquer, an organic adhesive, or an organic matrix resin, depending on the application. Anodizing improves adhesion between bonded metals, or between the metal and a fiber-reinforced composite in hybrid laminates, like those described in U.S. Pat. Nos. 4,489,123 or 5,866,272. We incorporate these patents by reference. Structural hybrid laminates have strengths comparable to monolithic metal, and have better overall properties than the metal because of the composite layers. At higher temperatures (like those anticipated for extended supersonic flight), conventional anodized treatments are inadequate in addition to being environmentally unfriendly. The thick oxide layers that anodizing produces become unstable at elevated temperatures. The oxide layer dissolves into the base metal to produce surface suboxides and an unstable interfacial layer.
Obtaining the proper interface for the organic resin at the surface of the metal is an area of concern that has been the focus of considerable research. For example, cobalt-based surface treatments are described in U.S. Pat. Nos. 5,298,092; 5,378,293; 5,411,606; 5,415,687; 5,468,307; 5,472,524; 5,487,949; and 5,551,994. U.S. Pat. Nos. 4,894,127 describes boric acid -sulfuric acid anodizing of aluminum.
Resins bond to a surface through bonding sites using covalent bonds, hydrogen bonds, or van der Waals forces. A coupling agent between the resin and metal often is required to improve adhesion. The present invention improves adhesion by placing a sol-gel film at the interface between the metal and resin. In effect, the sol-gel provides a metal-to-resin gradient through a monolayer of organometallic coupling agents. Generally we use a mixture of metals in the coupling agents. The organometallic compounds in our earlier applications, like the sols described in U.S. Pat. No. 5,849,110, preferably used zirconium and silicon organometallic compounds to interact with, react with, or bond to the metal surface. The Zr and Si also form a network in the sol-gel coating itself. Some mechanical interaction may result from the surface porosity and microstructure. The organic portion of the organometallic compounds here and in our previous sols usually has a reactive functional group for covalently bonding with the adhesive or matrix resin. Our organometallic coupling agent preferably bonds covalently with the metal and covalently with the resin. Thus, the sol-gel process has an oriented metal-to-resin gradient on the surface.
Situations involving extended exposure to hot/wet conditions (where polyimide adhesives hold promise) do not allow the adhesion surface preparation to continue to be the standard anodize processes or oxide surface preparations, especially for titanium. At high temperatures, the solubility of oxygen in titanium is high and the oxide layer simply dissolves with the oxygen permeating through the base metal. The result is interfacial failure at the metal-adhesive interface. To alleviate this type of bond failure, the surface oxygen needs to be tied up in a stronger bond that will not dissociate in bonding or during operation of the system. A zirconate-silicate sol coating of was useful at these extended hot/wet conditions because the Zrxe2x80x94O bond that forms between the coating and the metal surface is stronger than a Tixe2x80x94O bond. Both Zrxe2x80x94O and Tixe2x80x94O bonds are stronger than Sixe2x80x94O bonds. The higher bond strength prevents dissolution of the oxide layer, so the Zr component in our sol coating functions as an oxygen diffusion barrier. In the present invention, we use yttrium, in addition to or as a replacement for the Zr. Yttrium produces high strength oxide bonds that function as an oxygen diffusion barrier. The high cost of these compounds, however, dictates that they be used sparingly. Hence, we have developed another coating to produce the desired metal-to-resin gradient needed for good adhesion in structural adhesive bonds, hybrid laminates, or paint adhesion applications. The sol-gel coating integrates the barrier function of the Y (or, perhaps, Ce or La) with an organosilicate network desirable for superior bonding performance.
The present invention is a surface treatment for metal surfaces, especially aluminum or titanium alloys, using organometallics containing silicon and yttrium to produce a Sixe2x80x94Y sol-gel film surface coating, similar to the Zrxe2x80x94Si sol-gels of our earlier patents, suitable as an interface to improve adhesion between the metal surface and an organic matrix resin or adhesive. The sol-gel film or sol coating may improve corrosion resistance to a limited degree. It promotes adhesion of an organic resin to the metal through a hybrid organometallic coupling agent at the metal surface in a manner similar to our silicon-zirconium sol-gels. The sol is preferably a dilute solution of an yttrium alkoxide, such as yttrium propoxide or yttrium methoxyethoxide, and an organosilane coupling agent, such as aminophenyltrimethoxysilane.
The sol-gel film is applied by immersing, spraying, or drenching the metal in or with the Si-Y sol without rinsing. Key to the sol-gel film are bonding sites with the metal and separate sites to bond (or otherwise affiliate) with the resin. The sol-gel film produces a gradient changing from the characteristics of metal to those of organic resins. Good adhesion results from clean, active metal surfaces with sol coatings that contain the organometallic coupling agents in the proper orientation. After application, the sol coating is dried at ambient temperature or, more commonly, heated to a temperature between ambient and 250xc2x0 F. to complete the sol-gel film formation. The atomic ratio of Si:Y preferably ranges from 10:1 to 36:1, so these sols have a much higher concentration of Si relative to the other organometallic component than the Sixe2x80x94Zr sol. The Sixe2x80x94Y sol has utility and performance similar to the Sixe2x80x94Zr sol, at least for titanium substrates.
Ideally, covalent bonding occurs between the metal surface and an yttrium compound in the sol. Successful bonding requires a clean and chemically active metal surface. The strength and durability of the sol coating depends upon chemical and micro-mechanical interactions at the surface involving, for example, the metal""s porosity and microstructure and on the susceptibility of the sol coating to rehydrate. Our preferred sol coating should provide high temperature surface stability for paint adhesion, adhesive bonding, or fabrication of structurally superior hybrid laminates.